Probing the Dynamics of Electric Double-Layer Formation Over Wide Time Scales (10^-9 – 10⁺⁵ s) in the Ionic Liquid DEME-TFSI

Ionic liquids (ILs) have found wide use as the gate dielectric in a variety of novel transistors, allowing large carrier concentrations to be induced. These concentrations arise from the formation of an electric double layer (EDL) at the interface between the liquid and the channel, a process that yields an extremely high effective areal capacitance. ILs have been used to gate a variety of different materials, including polymers, oxides and low-dimensional materials, and have led to the discovery of exciting fundamental phenomena. In this work, we investigate the transient response of DEME-TFSI [N, N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide] based ionic-liquid (IL) planar capacitors, studying this response over time scales as short as a few nanoseconds, to as long as several days. Our measurements point to the existence of three distinct mechanisms for charging/discharging of the IL. The fastest of these is associated with the development of a standard polarization charge in the bulk of the liquid dielectric, which dominates at times shorter than ~10^-6 s. The second process is attributed to electric double layer formation, which is initiated after ~10^-6 s but which takes as long as ~10^-2 s to reach completion. Finally, we also identify the presence of a pseudocapacitance that arises from electrochemical reactions; this process is only activated at voltages above ~2.5 V and is relatively slow. Indeed, we find evidence that full discharging of this pseudocapacitance can take as long as 10⁵ s (i.e., days). Overall, our findings provide useful insight into the mechanisms for slow ion dynamics in ILs, and highlight the constraints that these dynamics place on the potential operational speed of IL-based transistors.